This invention relates to a coating composition, its preparation and use. Metal food and drink containers, often referred to as cans, are usually coated on the inside to prevent reaction between the contents and the metal from which the can is formed. Such reaction leads both to unwanted deterioration of the can and also potentially damaging effects on the contents, particularly in terms of changes in quality and taste. Without an interior coating, most cans of food or drink would not remain usable for very long. The coating is often applied to the flat metal by roller coating before the can is formed and then dried or cured in a stoving operation. The can is then formed from the flat metal by a drawing process before being filled with food or drink and finally sealed up.
The coatings are required to have very good flexibility, adhesion, sterilisation resistance and stability properties. Flexibility and adhesion are essential if the coating is to remain intact during the can formation process when the coated flat metal sheet is drawn into the form of the can. When the cans are filled with food, the contents are usually sterilised by heating the sealed can to temperatures of around 130xc2x0 C. for 1 to 2 hours (depending on the nature of the food). The coating is then in direct contact with the contents of the can for a considerable period of time which could be many years. During sterilisation and subsequent storage, the coating is required to maintain its integrity so as to prevent corrosion of the metal can and to prevent metal migration into the can contents. Additionally, the coating must not impair the contents by releasing unwanted material or by altering the flavour or appearance. These resistance properties impact not only on the shelf life of the product but also on public health and safety. Thus, there are particularly stringent and specific requirements of coating compositions for can interiors which are different from those for other coatings.
One known type of coating composition for cans is based on an epoxy resin. Epoxy resin compositions comprise an epoxy resin and, optionally, a crosslinker such as a phenolic resin dissolved or dispersed in an organic liquid. In those compositions containing a crosslinker, the crosslinker reacts with the epoxy groups on the epoxy resin during the stoving operation so as to form a crosslinked final coating. In known compositions the epoxy resin contains bisphenol A diglycidyl ether (abbreviated to BADGE), a commonly available liquid epoxy resin of low epoxy equivalent weight. Health concerns have arisen over the level of BADGE appearing in food supplied in cans which have been coated on the inside with epoxy coatings which invariably may contain some unreacted BADGE. Low molecular weight BADGE exists at high levels in low molecular weight commercial epoxy resins while low level of BADGE exists in high molecular weight epoxy resins. The concern is that all of the BADGE does not react with the crosslinker and that some of the residual free BADGE can leach out of the coating and into the food. As a result of these concerns, a limit on the level of free BADGE in the final cured coating for the interior of food cans has been proposed based on the amount of free BADGE in the coating and an assumption that all of this could theoretically migrate into the food. The current proposal is a limit on the quantity of free BADGE in the coating such that the contents would contain no more than 1 part per million (ppm) of BADGE if all of the BADGE were to migrate from the coating to the contents. This very low level of free BADGE is not easy to achieve by simple modifications of the existing formulations. The problem is particularly acute in smaller cans which have a larger interior surface area, and thus more coating, in relation to the volume of contents. The problem is to formulate coatings suitable for cans which meet the requirements for very low (less than 1 ppm) or zero (non-detectable) levels of BADGE or similar low molecular weight epoxy-based materials, appearing in food, while retaining or improving on all the other required characteristics of flexibility, adhesion and sterilisation resistance.
European Patent Application EP-A-0 111 986 discloses pigmented coating compositions based on an epoxy-polyester block copolymer in which the polyester component is prepared by polycondensation of terephthalic acid and/or isophthalic acid and a difunctional hydroxy compound having 2-24 carbon atoms. European Patent Application EP-A-0 399 108 discloses similar compositions in which the polyester is the condensation product of a carboxylic diacid and a dihydroxy compound in which the components are non-aromatic. However neither of these types of polymer are suitable for use in can coatings because neither gives the required cured film combination of flexibility, adhesion and sterilisation resistance and compatibility. In accordance with this invention, the cured film problems have been resolved along with extremely low levels of free BADGE by the use of a particular epoxy-polyester block copolymer in combination with fatty acids.
According to the present invention, provided is a coating composition comprising an organic liquid carrier in which is dispersed or dissolved a mixture of organic film forming components comprising by weight:
i) from 50% to 100% of an epoxy-polyester block copolymer consisting of the reaction product of an epoxide terminated epoxy resin and a preformed carboxyl functional polyester polymer,
ii) from 0% to 10% of organic monocarboxylic acid, preferably fatty acid, and
iii) from 0% to 50% of a crosslinker,
where the sum of (i) and (ii) is 100%, characterised in that the preformed polyester polymer is the reaction product of one or more polyols, predominantly diol, with dicarboxylic acid or their anhydrides, where the dicarboxylic acids comprise by weight (a) 20% to 45% of an aromatic polycarboxylic acid or its anhydride, (b) 55% to 80% cyclohexane dicarboxylic acid, and (c) 0% to 10% other difunctional carboxylic acid, where the sum of (a), (b) and (c) equals 100%, and where the epoxy-polyester is optionally further reacted with organic monocarboxylic acid.
It has been found that this use of a combination of an aromatic polyfunctional carboxylic acid or its anhydride and cyclohexane dicarboxylic acid in making the polyester gives rise to unexpectedly improved properties in the final film, particularly better adhesion, sterilisation and flexibility when compared to the use of either aromatic or aliphatic acids alone to produce BADGE-free or low-BADGE (containing less than 1 ppm) can coating compositions.
The resulting epoxy-polyester copolymer is the reaction product of an epoxy resin with a carboxyl functional polyester consisting of residue of the carboxyl functional polyester esterified with the residue of the epoxy resin, where the copolymer contains between 1 and 20 and preferably between 1 and 10 ester polymeric units. In preferred aspects of the invention, organic aliphatic monocarboxylic acid, preferably fatty acid, is further reacted with the epoxy-polyester polymer. The organic liquid carrier can be one or more organic liquids in which the epoxy-polyester block copolymer can be dissolved or dispersed. Typical organic liquids are aromatic solvents commercially available as Solvesso 100xe2x96xa1 or Solvesso 150(trademark) from Exxon.
Referring now to the epoxy prepolymer, suitable epoxy resins are aromatic or aliphatic epoxy resins with aromatic epoxy resins being preferred. Useful epoxy resins are predominantly linear chain molecules comprising the coreaction product of polynuclear to dihydroxy phenols or bisphenols with halohyrdrins to produce epoxy resins containing preferably two epoxy groups per molecule. The most common bisphenols are bisphenol A, Bisphenol F. bisphenol S and 4,4xe2x80x2-dihydroxybisphenol, with the most preferred being Bisphenol A. Halohydrins include epichlorohydrin, dichlorohydrih and 1,2-dichloro 3-hydroxypropane, with the most preferred being epichlorohydrin. Preferred epoxy resins comprise the coreaction product of an excess of halohydrin with bisphenol to produce predominantly an epoxy group (epoxide) terminated linear molecular chain of repeating units of diglycidyl ether of bisphenol-A containing between 2 and 25 repeating copolymerised units of diglycidyl ether of bisphenol-A. In practice, excess molar equivalents of epichlorohydrin are reacted with bisphenol-A to produce epoxy resins where up to two moles of epichlorohydrin coreact with one mole of bisphenol-A, although less than complete reaction can produce difunctional epoxy resin along with monoepoxide chains terminated at the other end with a bisphenol-A unit. The preferred linear epoxy resins are polyglydicyl ethers of bisphenol-A having terminating 1,2-epoxide groups and an epoxy equivalent weight between 150 and 5,000, preferably between 150 and 2,000, and a number average molecular weight from about 200 to 10,000, preferable from 200 to 5,000, as measured by gel permeation chromatography (GPC). Commercial diglycidyl ether of bisphenol-A (liquid epoxy) can be reacted with additional bisphenol-A to advance the epoxy and increase the molecular weight. The desired molecular weight and final oxirane content is controlled by adjusting the ratio of the two components and the extent of the reaction. Commercially available epoxy resins include Dow Chemical epoxy resins identified by trade number and equivalent weights as follows: DER 661(525); DER 664(900); DER 667 (3600); and DER 668(5500); while Shell Chemical epoxy resins are EPON 1001(525); EPON 1007(2000); EPON 1009F (3000); EPON 1007F (4000); and EPON 1009(6500); and Ciba-Ciegy linear epoxy reins GT-7013(1400); GT-7014(1500); GT-7074(2000); and GT-259(1200). Although not as common, trifunctional epoxy resins are useful comprising branched chain epoxy resins where the branched chains as well as the backbone chain are each terminated with a terminal epoxide groups to provide greater than two epoxide functionality. Trifunctional epoxy resins can be produced by coreacting epichlorohydrin with polynuclear polyhydroxy phenols, trifunctional phenols, or aliphatic trifunctional alcohols.
Useful epoxy resins further include non-aqueous alkylene oxide resins which are epoxide functional resins comprising an alkylene oxide adduct of a bisphenol compound. The alkylene oxide is an aliphatic alkyl derivative having up to about 26 carbon atoms although preferred oxides are lower alkyl oxides such as ethylene, propylene, and butylene oxides. Bisphenol compounds include bisphenol-A, bisphenol-F and bissulfone or sulfides. Typically two or more moles of alkyl oxide are coreacted with one mole of bisphenol compound. Preferred compositions are 2:1 molar reactions while suitable number average molecular weight range of alkylene oxide resins is between 200 and 1,000 as measured by GPC. The most preferred preformed epoxy resin contains two epoxy groups per molecule and the preferred epoxy is based on bisphenol-A.
Referring next to the preformed carboxyl functional polyester, the polyester comprises polyol, predominantly or entirely a glycol or a diol, esterified with excess equivalents of dicarboxylic acid or anhydrides including considerable amounts of cyclohexane dicarboxytic acid. On a weight basis, the polyester comprises between 55% and 80% cyclohexane dicarboxylic acid, between 20% and 45% aromatic dicarboxylic acid or anhydride, and between 0 and 10% other aliphatic dicarboxylic acid. Useful polyfunctional alcohols have two or more hydroxy groups where the predominant polyol contains two hydroxyl groups. Examples of suitable polyols include ethylene glycol, 1,4-butane diol, 1,2-propylene glycol, 1.3-propylene glycol, methyl propane diol, neopentyl glycol, 1,6-hexane diol, butyl ethyl propane diol, hydroxy pivolyl hydroxy pivalate, cyclohexane dimethanol, trimethylol propane, pentaerythritol and glycerol. 1,4-butane diol is preferred. Minor amounts of glycerol, pentaerythritol, dipentaerythritol or trimethylolethane or propane can be used if desired. Examples of suitable aromatic dicarboxylic acids are phthalic acid, terephthalic acid or isophthalic acid with isophthalic acid particularly preferred. The anhydride derivatives of these acids an also be used if they exist as anhydrides. The dicarboxylic acid content of the polyester prepolymer further comprises between 55% and 80% cyclohexane dicarboxylic acid. Preferably less than 10% by weight of the dicarboxylic acid content comprises other aliphatic dicarboxylic acids. examples of other aliphatic polyfunctional carboxylic acids are malonic acid, succinic acid, glutaric acid, adipic acid, azelaic acid, suberic acid, sebacic acid, dimer fatty acids, maleic acid and dimer fatty acids. Hydroxy acids can also be included in the polyester such as 12 hydroxy stearic acid. lactic acid and 2-hydroxy butanoic acid. To provide a carboxyl terminated polyester, the equivalent excess or dicarboxylic acid over polyol is between 0.02 and 0.784, and preferably between 0.04 and 0.554. The acid number of the preformed polyester should be greater than 5 and preferably between 10 and 140 mgKOH/g.
The preformed polyester can be made by heating the components together at a temperature of 150 to 280xc2x0 C., preferably 200 to 250xc2x0 C. and removing the water evolved for example by azeotropic distillation with the aid of an organic solvent such as toluene or xylene. Heating is continued until the polyester has an acid number preferably between 10 and 140 mKOH/g, more preferably 20 and 110. A catalyst can be used to speed up the esterification reaction. suitable catalysts are acid catalysts such as sulphuric acid, paratoluene sulphonic acid or tin catalysts, such as dibutyl tin dilaurate. The hydroxyl number of the polyester is preferably no higher than 2, and more preferably 0 to 0.8. The number average molecular weight of the preformed polyester is preferably 800 to 15 000, and more preferably 800 to 8000.
In accordance-with this invention, the epoxy-polyester block copolymer can be prepared by heating the acid functional polyester polymer and the epoxy resin together at a temperature of 110 to 180xc2x0 C, preferably 120 to 170xc2x0 C. for 1 to 5 hours, preferably 2 to 4 hours. A catalyst for the carboxyl/epoxy reaction can be included such as triphenyl phosphine, benzyl triphenyl phosphonium chloride, benzyl trimethyl ammonium methoxide, a tertiary amine such as benzyl dimethylamine or a metal compound such as zirconium octoate. The reaction can be carried out in a suitable solvent such as toluene or xylene. The weight ratio of the polyester and the epoxy resin is preferably chosen so that the equivalent ratio of epoxy groups from the epoxy resin to acid groups from the polyester is 1:2 to 2:1, more preferably 1.6:1 to 1:1.6. The resulting epoxy-polyester block copolymer preferably has a number average molecular weight of 3 000 to 40 000, and a low Acid Number below 10 and preferably less than 1 mgKOH/g.
In a preferred aspect of this invention, the epoxy-polyester block copolymer is further reacted with an organic monocarboxylic acid, preferably an aliphatic monocarboxylic acid, and most preferably an aliphatic fatty acid having a fatty acid chain of 8 to 24 carbon atoms. The most preferred fatty acids ordinarily are derived from vegetable oils or fats which conventionally contain a mixture of glyceride oils comprising glycerol esters of fatty acid. Fatty acids are typically obtained by hydrolysis of vegetable oils or fats. Useful fatty acids include lauric acid, capric acid, palmitic acid, stearic acid, oleic acid, linoleic acid, linolenicacid, eleostrearic acid and ricinoliec acid. The preferred fatty acid is coconut fatty acid. Less preferred lower aliphatic monocarboxylic acids can include formic acid, acetic acid, propionic acid, n-butyric acid, isobutyric acid, n-valeric and isovaleric acids, pivalic acid, and capronic acid. less preferred aromatic monocarboxylic acid include benzoic and toluic acids. In accordance with this invention, the organic monocarboxylic acid is post reacted with unreacted epoxide associated with the prior formed epoxy-polyester block copolymer.
Surprisingly it has been found that the levels of free epoxy resin (particularly low molecular weight resin such as BADGE) in the final composition can be further reduced by heating the block copolymer in the presence of an acid or a base, of which phosphoric acid has been found particularly effective. The copolymer can be heated in the presence of the phosphoric acid or base for example for 30 to 240 minutes at 100 to 200xc2x0 C., preferably 1-3 hours at 120 to 180xc2x0 C. Preferred bases are dimethyl benzylamine, diethylene triamine, and tributyl amine. The acid or base can be added at levels of, for example, 0.2 to 10% by weight based on the polymer solids, preferably 0.5 to 10%. Such treatment can reduce the level of free epoxy resin below normally detectable limits.
Preferably the composition comprises at least 1% by weight of crosslinker and more preferably comprises from 50 to 97 parts by weight of epoxy-polyester block copolymer and 3 to 50 parts by weight of crosslinker. The crosslinker is a material which will react with the epoxy polyester to give rise to a crosslinked final coating. Examples of suitable crosslinkers are phenolic resins, amino resins and blocked polyisocyanates. Preferably the crosslinker is a phenolic resin. Phenolic resins are reaction products of phenol compounds with formaldehyde, and are divided into resols in which the reaction is base catalysed and novolacs in which the reaction is acid catalysed. When a cross linker is used, the coating compositions can also contain a catalyst for the reaction between the remaining epoxy groups on the epoxy polyester polymer and the phenolic resin such as phosphoric acid. The compositions can be made by mixing the components in any order, such as by adding the epoxy-polyester block copolymer and the crosslinker to the organic liquid carrier.
The compositions also preferably contain finely divided PVC, in which case the liquid medium is chosen so as to dissolve the PVC polymer to only a minor extent or not at all. Such compositions are referred to as PVC organosols. PVC organosols are particularly known for their flexibility and sterilisation resistance properties and are used for cans containing some of the more aggressive products, such as acid-containing foods. They are also used for xe2x80x9ceasy open endsxe2x80x9d, that is to coat the inside of food or drink cans is which the metal (usually the lid, or part of the lid) is partially cut through during manufacture to facilitate opening of the can by the consumer using a ring-pull or similar opener. Where PVC is used, the epoxy resin is found to stabilise the PVC polymer against decomposition during curing of the composition after it is applied to the metal. Useful PVC is preferably finely divided polyvinyl chloride in powder form is commercially available from a number of sources. Preferably the PVC powder has a particle size range of 0.5 to 12 xcexcm. Examples of suitable commercially available PVC powders are Geon 171(trademark) (from the Geon Company) and Vinnol P70(trademark) from Wacker Chemie). Preferred compositions comprise 10 to 80 weight % of PVC based on the total non-volatile weight of epoxy-polyester block copolymer, crosslinker and PVC, more preferably 20 to 70 weight %. The PVC is preferably added to the compositions by what is known as a grinding process using a ball mill or sand mill.
Preferably the compositions are pigment free when they are for use in coating the interior of cans. Pigments tend to be a source of weakness in such coatings and are often detrimental to their performance. Other components of the compositions can be waxes and flow additives as well as other conventional coating components. The compositions preferably contain less than 1 ppm of BADGE, more preferably less than 0.5 ppm and most preferably below detectable limits.
The compositions can be applied as coatings to a variety of substrates such as plastic, glass or metal but are particularly suited to coating metal. In particular they are useful in coating food or beverage cans, especially the interiors of such cans where their low or undetectable BADGE content and their other properties make them particularly desirable. The composition can be applied as a film by conventional means such as brushing, roller coating or spraying. Roller coating is the preferred method when coating flat metal for can manufacture and spraying is preferred when coating preformed cans.
The applied film of the composition can be dried and cured by heating to drive off the organic liquid and to accelerate the crosslinking reaction between the epoxy-polyester and the crosslinker. The composition is typically heated to 150-220xc2x0 C. for 1 to 20 minutes in order to from a dried, cured film. A so-called xe2x80x9cflash-stoving, i.e 10-30 seconds at a peak metal temperature of 220-260xc2x0 C. can also be used.
Another type of stoving is induction curing i.e. 4-6 seconds at a peak metal temperature 280-320xc2x0 C. PMT. Compositions were evaluated on tinplate for use on easy-open ends. Easy open ends require cured coatings exhibiting high flexibility and sterilisation resistance, as well as the normal tests, wedge bend drawn cans and ends. Good resistance to feathering which is the uneven tearing of the film at the easy open end edge on opening is also required. A test for feathering is as follows:
A 5 cmxc3x9710 cm panel coated with the composition is placed on a flexible supportxe2x80x94rubber pad, wads of paper. The internal coating to be evaluated is in contact with the support. On the back of the panel, a metal ruler is placed and the metal is scored with a scalpel longitudinally, perpendicular to the metal grain at a distance of about 1-1{fraction (1/2 )} cm from the edge. There is now a longitudinal bump on the internal coating. The metal is then cut with scissors about 1 cm along the groove/bump and the approximately 1{fraction (1/2 )} cm width is gripped in a sardine can opener and the strip is rolled slowly around the openerxe2x80x94exactly as when opening a can. The metal panel is then turned over and the edge of the flat part examined with the naked eye and with a magnifying glass. Ideally the interior coating should not go beyond the metal. A slight amount of varnish detached from the rolled part is toleratedxe2x80x94width 0.5 mm. This varnish should be a uniform strip, and not present a feathered appearancexe2x80x94which is indicative of detached varnish particles which could fall into the food/beverage. This method is a severe control, since if done quickly i.e.. at the speed of opening a can, then the cut edge should be perfect. This test is carried out on non-sterilised and sterilised panels.